Closed anode fuel cell startup method

ABSTRACT

A process for starting a PEM fuel cell module includes blowing air through the cathode side of the module using external power. An amount hydrogen is released into the anode side of the module under a pressure greater than the pressure of the air on the cathode side, while the anode is otherwise closed. Cell voltages in the module are monitored for the appearance of a charged state sufficient to start the module. When the charged state is observed, the module is converted to a running state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application of U.S. ApplicationSer. No. 62/437,421, filed Dec. 21, 2016, which is incorporated byreference.

FIELD

This specification relates to fuel cell power modules for example PEMfuel cell modules.

BACKGROUND

In one exemplary process for shutting down a PEM fuel cell module, thecathode side of the module is vented while the anode side is closed butreceives hydrogen from a small shut down reservoir (i.e. less than 5 L)at a small pressure (i.e. less than 5 psi above atmospheric pressure)until the reservoir is empty. The anode and cathode sides are connectedelectrically through a resistor. Oxygen on the cathode side is consumedthereby blanketing the cathode side with nitrogen. Hydrogen on the anodeside is also consumed creating a vacuum that draws nitrogen over to theanode side. The fuel cell module is thereby blanketed with nitrogen. Thecapacitive charge of the cells is also discharged through the resistor.If the module is left for a long time, some or all of the nitrogen maybe replaced with air. Examples are described in WO2004114448 and WO2007090284, both entitled Passive Electrode Blanketing in a Fuel Cell.

To re-start the module, a small fan powered by a battery or other energysource is operated to displace nitrogen on the cathode side with air.Hydrogen is introduced into the anode side of the module from a largerfuel reservoir and dispersed throughout the anode side of the modulewith a recirculation pump or by opening a purge valve. Once the cells ofthe module regain sufficient charge, the module can sustainelectrochemical reactions sufficient to run its balance of plant and themodule is considered to have restarted.

INTRODUCTION

After a shut down procedure, for example as described above, the anodeand cathode sides of a fuel cell may be blanketed in nitrogen or mixtureof gasses (i.e. air) that is primarily nitrogen. The mixture may have,and typically does have, more nitrogen than ambient air, possibly near100% nitrogen. Further, the gas diffusion layers on either side of themembranes in a PEM fuel cell may inhibit the replacement of nitrogenwith oxygen and hydrogen during the start up procedure.

The MEAs in a fuel cell stack are inherently capacitive. Re-starting afuel cell module requires all cells to reach at least a partiallycharged state before the stack is used to supply a material current. Toachieve this charged state, hydrogen and oxygen must be dispersedthroughout the anode and cathode sides of the stack. Hydrogen can bedispersed with a hydrogen recirculation pump in a loop around the anodeside, but that is a complication that consumes stored power or ifpowered from the stack can delay or prevent startup depending on thehydrogen and air concentration throughout the fuel cell stack and theindividual cell voltages within the fuel cell stack. Alternatively,hydrogen can be dispersed by flowing hydrogen through the anode side ofthe module to a vent or purge valve on the outlet of the anode side.This releases hydrogen to the atmosphere, which is not a pollutant butcan in some cases create a combustible mixture outside of the vent. Thepresence of a combustible mixture can be prevented by venting hydrogeninto the cathode side exhaust and blowing sufficient air through thecathode side. However, the airflow rate required for dilution exceedsthe airflow rate required to charge the stack, so this method consumesmore stored power.

In some cases, the procedure described above may also fail to start amodule that is very old or damaged (i.e. by environmental contamination)and so has membranes that are more permeable than normal.

It is desirable in at least some circumstances to minimize the amount ofstored power (i.e. in an external battery) needed to start a fuel cellmodule. It is also desirable in at least some circumstances to avoidventing hydrogen, at least before a large flow of air is available todilute the hydrogen. It is also desirable in at least some circumstancesto be able to restart old or damaged fuel cell modules.

This specification describes a process for starting a fuel cell module.Optionally, the process can be implemented without purging hydrogen orrecirculating hydrogen through the anode side of a module. The processcan also be used to start a module with old or degraded membranes thatare more permeable than new membranes. In the process, hydrogen isdistributed through the anode side of the module. The pressure on theanode side of the module is adjusted such that the cells of the modulereach at least a minimum voltage. The module is then converted to arunning state.

In some examples, the process includes blowing air through the cathodeside of the module using a small amount of external power, for exampleless than 1000 W or less than 100 W. Hydrogen is released into the anodeside of the module, preferably under a pressure greater than thepressure of the air on the cathode side, and preferably in an amount andpressure effective to distribute hydrogen to substantially all of thecells. The distribution of hydrogen is aided by the permeation of gasesfrom the anode side to the cathode side driven by the differentialpressure. The pressure differential across the membrane is then reduced,optionally by releasing only a finite amount of pressurized hydrogenwhile the anode side is otherwise closed in the previous step such thatthe permeation of gasses to the cathode side inherently reduces pressureon the anode side over time. Cell voltages in the module are monitoredfor the appearance of a charged state sufficient to start the module,i.e. to power the balance of plant from the fuel cell stack rather thanexternal power. When the charged state is observed, the module isconverted to a running state. In the running state, a generallycontinuous supply of hydrogen is provided to the anode side and thestack is used to power a higher air flow through the cathode side.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic drawing of parts of a fuel cell module.

DETAILED DESCRIPTION

Although the anode side and cathode side of a PEM fuel cell module areseparated by membranes, there is always some flow of gas through themembrane in a PEM fuel cell. The membrane is typically less than 20microns thick and will permeate nitrogen, hydrogen or air under theinfluence of a pressure or concentration differential across themembranes. In normal operation the presence of hydrogen on the cathodeside of the module is typically considered preferable to having air onthe anode side. Hydrogen permeation across the membrane may causecombustion on the cathode side, but the resulting water can be removed,or non-combusted hydrogen diluted, by blowing air through the cathodeside while the module is running. Air permeation to the anode side ofthe stack can lead to nitrogen accumulation on the anode, which wouldrequire additional purging to prevent a concentration buildup thatimpacted fuel cell performance. Accordingly, some modules supplyhydrogen through a forward pressure regulator biased to provide hydrogenat a small pressure, i.e. less than 5 psi or less than 1 psi above thepressure in the cathode side. As a module ages or is exposed toenvironmental contamination, the permeability of the membranesincreases.

The cells in a fuel cell stack (in particular the MEA) inherentlyoperate to some extent like capacitors. The capacitive charge (or simplycharge) of the stack, or a cell or group of cells in the stack, can bedetermined according to an equivalent circuit module or approximated bymeasuring the voltage of the cell, group of cells or stack. When amodule is off, the cells are typically discharged so as to present lessof a safety concern (i.e. that someone working on the cell stack couldreceive a shock). When the module is restarted, all cells in the stackmust develop a minimal charge (or voltage) before the stack can beallowed to deliver significant power (or current). Otherwise, there is apotential for the voltage in a weakly charged cell to become negative.This reversal of polarity could damage the cell. Therefore cell voltageis monitored and the stack is not allowed to provide material power, oroptionally any power, until all cells in the stack have at least aminimum charge (i.e. voltage), for example 25% or 50% of normal ormaximum cell voltage, or for further example 0.1 V or 0.2 V per cell forcells that normally (or nominally) operate at 0.5 V to 0.6 V. Cellvoltage can be monitored by monitoring the voltage of the stack, groupsof cells or individual cells. However, if only the voltage of the stackis measured a factor of safety is preferably applied (i.e. a largerminimum voltage per cell is required) to account for the risk that somecells have less than the average voltage per cell. The factor of safetycan be reduced if the voltage of groups of cells, i.e. 10 cells or lessor 5 cells or less, is monitored. Preferably, the voltage of groups ofcells or individual cells is monitored.

With very permeable membranes, supplying hydrogen even continuously atan ordinary pressure differential while blowing air at a low ratethrough the cathode might never create the required minimum charge ineach cell. The combustion of hydrogen on the cathode side consumesoxygen, leaving insufficient oxygen for the electrochemical reactionrequired to charge the cells. While it may be possible to start themodule by providing more air, powering a large air fan (for example thenormal operating blower) using external power is undesirable in at leastsome circumstances. The method described below can be used to startmodules with very permeable membranes without significant external power(i.e. other than a small battery typically provided to power controlelectronics). The method can also be used to start modules with normallypermeable membranes. In these cases, conventional startup proceduresinvolve venting hydrogen or recirculating hydrogen with a pump usingsignificant external power whereas the method described above does notrequire either. Some hydrogen is released to the cathode side, but therelease rate can be such that the hydrogen is combusted in a controlledmanner in the cathode side of the module such that only a small amountof hydrogen, or no hydrogen, is released to the atmosphere outside ofthe fuel cell module.

FIG. 1 shows a fuel cell power module 10. The module 10 has a fuel cellstack typically containing many cells, represented schematically in FIG.1 by anode side 12 and cathode side 14. A hydrogen tank 16 provideshydrogen to anode side 12 though inlet valve 18, shutdown reservoir 20and forward pressure regulator 22. An outlet from the anode side 12 canbe closed with a purge valve 24. The cathode side 14 receives air fromeither small blower 26, which is powered by an external battery or powersource, or large blower 28, which is powered by the stack or anotherexternal power source. Forward pressure regulator 22 may be a domeloaded pressure regulator with its dome connected to an inlet to thecathode side.

A resistor 30 is attached between the anode side 12 and cathode side 14for use during a nitrogen blanketing shut down procedure. Shutdownreservoir 20 is used, after closing inlet valve 18 and purge valve 24,to provide a quantity of hydrogen for the shutdown procedure. Forwardpressure regulator 22 maintains the pressure of hydrogen in the anodeside 12 at an increment, for example in the range of 0.1 psi to 5 psi,or 0.5 to 1 psi, above the pressure in the cathode side 14 as long assufficient hydrogen is available. Thus, during the shutdown method,shutdown reservoir 20 may contain a volume of hydrogen at 100 psi (orwhatever pressure, optionally regulated, is delivered from tank 16) andreleases the hydrogen over time through forward pressure regulator 22.After the shutdown procedure, the anode side 12 and cathode side 14 areat least partially blanketed with nitrogen and the electrochemical andcapacitive charge of the stack has been dissipated through resistor 30.

To restart the module, small blower 26 is operated, for example by anexternal battery used to power a controller and valves (typicallysolenoid valves) for the module 10. The external battery powering thesmall blower 26 may have a different nominal voltage than the stack, forexample the battery may have a nominal voltage in the range of 3-24volts whereas the stack has a nominal voltage of 48 volts or more. Purgevalve 24 is closed and a hydrogen recirculation loop, if any, is notpowered. Inlet valve 18 is opened temporarily to admit a finite amountof hydrogen into shutdown reservoir 20 and then closed. Hydrogen isreleased from the shutdown reservoir 20 to the anode side 12, optionallythrough the forward pressure regulator 22 which maintains a pressuredifferential across the membranes for as long as sufficient hydrogen isavailable in shutdown reservoir 20. The hydrogen diffuses through theanode side 12 and gasses (i.e. hydrogen and nitrogen) permeate to thecathode side 14. Permeation to the cathode side 14 eventually causes thepressure differential across the membranes to decrease as the shutdownreservoir is depressurized. As a result, the rate of hydrogen permeationacross the membranes also declines but after the pressure differentialhas usefully caused hydrogen to be dispersed through the anode side.Cell voltages are monitored during the procedure. When all cells (whichcan optionally be determined by monitoring groups of cells or the stack)show a sufficient charge or voltage (which has been observed togenerally coincide with a decrease in the pressure differential acrossthe membranes), the module 10 is converted to a running state.Conversion to the running state includes re-opening solenoid valve 18and powering large blower 28 from the stack or other external powersource.

As a result of the method describes above, hydrogen is provided on theanode side of the module initially under a first pressure. The firstpressure may be greater than the pressure of the air on the cathode sideby an amount determined by the forward pressure regulator. Later, aftersome of the hydrogen is consumed or hydrogen is otherwise dispersedthrough the anode side of the module, hydrogen is provided on the anodeside of the module under a second pressure, which may be a variablepressure. The second pressure is lower than the first pressure.Preferably the second pressure is still greater than the pressure on thecathode side of the module, or at least equal to the pressure on thecathode side of the module, at all times. Optionally, the secondpressure might briefly (i.e. for less than 1 second) be less than thepressure on the cathode side of the module. However, negative pressureon the anode side of the module is preferably not sufficient, in timeand duration, to draw a material amount of air to the anode side.

In an alternative method, inlet valve 18 is not closed but forwardpressure regulator 22 is adjusted to decrease the pressure differentialacross the membranes to provide the second pressure. For example, theforward pressure regulator 22 can be continuously adjusted whilemonitoring cell voltage until the cells show sufficient charge orvoltage to convert the module to the running state. In another example,a second pressure is predetermined, for example by observing a pressuredifferential that coincides with an increase in cell voltage using anyof the variable anode side pressure methods described herein, and theforward pressure regulator 22 is adjusted to provide the predeterminedsecond pressure. For example, the second pressure might be 0.5 psi orless or 0.25 psi or less. In some cases, a predetermined second pressuremight also be effective to also disperse hydrogen through the anode sideof the module 10 such that a distinct first pressure is not required.However, while this might be sufficient to start the module 10, it islikely to increase the time required to start the module 10 relative toa method with a higher first pressure.

The conversion to a running state is preferably done quickly, before thevoltage or charge in the cells is dissipated through the resistor 30.Alternatively, the resistor 30 can be disconnected during the startupprocedure to avoid or reduce the need to convert the module 10 to arunning state quickly upon observing sufficient cell voltage, or uponobserving a decrease in the pressure differential. Although the MEAs inthe stack are inherently capacitive, and can hold and release anelectric charge, they do not have sufficient storage capacity todischarge through resistor 30 for more than a few seconds and thenoperate large blower 28 for enough time to develop an electrochemicalreaction sufficient for sustained operation.

The inventors have observed that the cells do not develop a charge whilehydrogen is delivered to the anode side 12 and a pressure differentialis maintained across the membranes by the forward pressure regulator 22,at least when a permeable module 10 (i.e. a module 10 wherein themembranes in the fuel cell stack have developed a gas crossover leakfrom age, usage or environment contamination) is restarted using a smallblower 26. In the example illustrated, small blower 26 is an air flowmeter normally used when the module 10 is running to monitor theoperation of large blower 28 but powered for use in the start up method.This small blower 26 uses less than 50 watts, typically less than 20watts, and is operated from a small battery sized (in both voltage andwatt-hours) primarily to operate solenoid valves and a controller. Whilethe hydrogen pressure and permeation usefully distribute hydrogenthrough the anode side 12, without intending to be limited by theory, itis possible that hydrogen permeating across the membranes consumesenough of the available oxygen on the cathode side 14 in combustion thusleaving insufficient oxygen to generate an electrochemical reaction tocharge the cells. However, as the shutdown reservoir 20 empties withinlet valve 18 closed, the hydrogen pressure in the anode side 12decreases despite the action of the forward pressure regulator 22.Hydrogen permeation decreases while the small blower 26 continues tosupply fresh air to the cathode side 14. Eventually, conditions suitablefor the electrochemical reaction appear and the cells develop a charge.

While the process is described above a being useful to start old ordamaged modules, it can also be applied to new modules, perhaps withadjustments to the initial amount and pressure of hydrogen supplied. Inthe case of a new module, the method avoids venting or recirculatinghydrogen. Venting might not be permitted at all in some circumstances,or permitted only to the cathode side outlet in the presence of a largeairflow. Recirculating hydrogen by pump or providing a diluting flow ofair both require large amounts of power, which would require the module10 to be started by an external power supply or carry a significantlylarger battery.

In the example above, the cathode side 14 is essentially at atmosphericpressure. However, it is expected that the method would still work witha pressurized cathode side 14 provided that hydrogen was temporarilyprovided at an even higher pressure to the anode side. It is possible insome examples that the shutdown reservoir 20 is not required. Thefunction of the shutdown reservoir 20 might be provided by a pipebetween the inlet valve 18 and the anode side 12, or by a manifoldwithin the anode side 12 of a stack, or both. Alternatively oradditionally, the inlet valve 18 could be opened briefly multiple timesto admit pulses of hydrogen collectively providing an effective amountof pressurized hydrogen into the anode side. In another option, ratherthan reducing the anode side pressure after hydrogen is supplied to theanode side 12, the pressure of the cathode side 14 might be increased toprovide a similar reduction in the pressure differential. The overalleffect desired is to allow hydrogen (and possibly nitrogen) to permeatethrough the membrane for a period of time and then to decrease or stopthe hydrogen (and possibly nitrogen) permeation preferably whileproviding at least a small flow of air to at first combust permeatedhydrogen and then to provide an electrochemical reaction to charge thecells.

In the method described above, hydrogen is dispersed through the anodeside 12 by adding a quantum (i.e. mass or molar amount) of hydrogenunder pressure relative to the cathode pressure, and preferably alsorelative to the pre-existing anode pressure, into an otherwise closedanode. It is believed that allowing some hydrogen permeation through themembranes helps distribute hydrogen through the anode side 12. Thequantum of hydrogen added is preferably sufficient to create a hydrogenconcentration sufficient to charge the cells. 3% hydrogen may besufficient. Although the method as described above does not involveopening purge valve 24, opening purge valve 24 with inlet valve 18closed after delivering only a small amount of hydrogen to anode side 12could be used to depressurize the anode side 12 after hydrogen isdispersed. The amount of hydrogen that would be released through thepurge valve 24 would be much less than under a conventional startuppurge with inlet valve 18 open and might usefully reduce the timerequired for the start up procedure when used with a module 10 havingnew or otherwise minimally permeable membranes.

Closing the inlet valve 18 temporarily, although counterintuitive whentrying to start a module 10, or otherwise providing only a fixed quantumof hydrogen to the anode side 12, allows hydrogen permeation to thecathode side 14 to eventually cease or become small enough to allow forthe electrochemical reaction to occur and charge the cells. Cell voltagewill rise as the electrochemical reaction occurs and charges the cell.However, particularly if there is a shut down resistor 30 attached, thecharge will soon disappear. Therefore it may be useful to provide anautomated controller that monitors the cell charges and automaticallyruns the module 10 (i.e. re-opens inlet valve 18 and turns on largeblower 28 as soon as the least charged cell (or group of cells) reachesa predetermined minimum charged state).

When the startup method is used with a module 10 having minimallypermeable membranes, permeating the amount of hydrogen contained in atypical shutdown reservoir 20 could take several minutes or more. Inthat case, inlet valve 18 could be opened only very briefly such thatshutdown reservoir 20 is not completely pressurized and filled fromhydrogen tank 16. Alternatively or additionally, the pressuredifferential provided by the forward pressure regulator 20 could beincreased during the start up procedure. In another alternative, abypass line might be provided around the shutdown reservoir 20 such thatopening inlet valve 18 temporarily only provides as much hydrogen to beadded downstream of inlet valve 18 as forward pressure regulator 20 willallow. In this case, the bypass line would act to some extent as asmaller shutdown reservoir 20 and could be sized (or provided with asmall reservoir) accordingly. Alternatively, an optional purge valve 31can be used to more rapidly reduce the hydrogen pressure on the anodeside of the fuel cell and/or ensure adequate hydrogen distributionthroughout the fuel cell stack by introducing gas from the anode to thecathode side of the fuel cell whereby it is consumed by combustion(which may be catalytically facilitated) internally within the fuel cellstack.

In alternative methods, a different method (other than applying thefirst pressure) might be used to disperse the hydrogen. For example, asmall amount of hydrogen can be vented through the module or a hydrogenrecirculation loop can be powered briefly by a battery. After dispersingthe hydrogen, hydrogen is applied at a second pressure, i.e. a pressurelower than the ordinary operating pressure, optionally at a pressuredifferential of 0.5 psi or less or 0.25 psi or less, optionally at avariable pressure. The module is then converted to a running state.However, while these methods might usefully start a leaky module, theyare not as efficient as applying the first pressure to disperse thehydrogen.

We claim:
 1. A method of starting a fuel cell module comprising thesteps of: blowing air through the cathode side of the module using powerother than from the stack; providing hydrogen on the anode side of themodule under a first pressure greater than the pressure of the air onthe cathode side and later at a second pressure that is lower than thefirst pressure; monitoring the voltages of the cells in the module forthe appearance of a sufficiently charged state; and, after the precedingsteps and the appearance of a sufficiently charged state, converting themodule to a running state.
 2. The method of claim 1 wherein air is blownthrough the cathode at a rate that is 20% or less of the lowest rateused in normal running conditions, or by a fan smaller than the fan usedunder normal running conditions.
 3. The method of claim 1 wherein afinite amount of hydrogen is released into communication with the anodeside of the module.
 4. The method of claim 3 wherein the hydrogen isreleased through a forward pressure regulator.
 5. The method of claim 3wherein the anode side of the module is closed.
 6. The method of claim 1wherein the sufficiently charged state is indicated by the least chargedcell be sufficiently charged to not have a negative voltage when themodule is run.
 7. The method of claim 1 wherein the sufficiently chargedstate is indicated by the least charged cell being above a thresholdcharge.
 8. The method of claim 1 wherein the running state ischaracterized by the fuel cells of the module powering at least thebalance of plant.
 9. The method of claim 8 wherein the running state ischaracterized by the module being provided with a continuous supply ofhydrogen.
 10. The method of claim 1 wherein the second pressure is avariable pressure.
 11. The method of claim 5, wherein the anode side ofthe module is closed without being vented and/or without recirculatinghydrogen.
 12. The method of claim 7, wherein the threshold charge is25-50% of a fully charged state.
 13. The method of claim 8, wherein thefuel cells of the module power at least the balance of plant using anair blower in communication with the cathode side.